Magnetic lenses

ABSTRACT

Apparatus having a generator of a beam of charged particles and magnetic focusing means in which a beam of charged particles is brought to a focus between the source and the focusing means so that radiation from a target placed at the focus can be received by a radiation detector within a substantial solid angle, bounded by said magnetic focusing means and the beam of charged particles. Preferably the focusing means is an electrically conducting coil of substantially flat or shallow conical form having a conical half-angle of not less than 75*.

o l-@5 72 XR 3,659,097

United States Patent 1151 3,659,097 Bassettetal. [4 1 M11225, 1972 [54]MAGNETIC LENSES 3,008,044 11/1961 Buchhold ..250/49.5

3,189,953 6/1965 Smith,.lr. .219/121EBX [721 lnvemms Rich! 3 9*Cheltenham; Thlmas 3,417,224 12/1968 Steigerwald et al... ....219/121 EBBlrmmgham, bmh Englarld 3,437,734 4/1969 Roman et al ..219/121 EB x [73]Assignee: National Research Development Corporai L d E l d PrimaryExaminer-William F. Lindquist Attorney-Cushman, Darby & Cushman [22]F1led: Feb. 16, 1971 g [21] Appl. No.: 115,847 ABSTRACT RelatedApplication Data Apparatus having a generator of a beam of chargedparticles and magnetic focusing means in which a beam of charged par-[63] Continuation of Ser. No. 781,219, Dec. 4, 1968, abanticles isbrought to a focus between the source and the focusdoneding means sothat radiation from a target placed at the focus can be received by aradiation detector within a substantial 219/121 solid angle, bounded bysaid magnetic focusing means and the 250/495 PE beam of chargedparticles. [51] Int. Cl. ..l-l0lj 37/26 58 Field of Search ..219/121 EB;250/495 A, 49.5 B, Preferably the focusmg means 18 an elecmcallyconductmg 250 495 D, 9 5 313/34 coil of substantially flat or shallowconical form having a conical half-angle of not less than 75.

[56] References cued 11 Claims, 7 Drawing Figures UNITED STATES PATENTSI 2,058,914 10/1936 Riidenberg ..250/49.5

PATENTEDAPR 2 5 I972 sum 2 OF 3 FIG4 I I I I I I I I f iv Invmtar 8Horny;

PATENTEDAPR 25 m2 SHEET 3 OF 3 FIG] ' MAGNETIC LENSES This applicationis a continuation of application, Ser. No. 781,219 filed on Dec. 4,1968, now abandoned.

. This invention relates to magnetic lenses for the focusing of beams ofcharged particles, in particular of beams of electrons.

It is well known to focus a beam of electrons by causing it to passaxially through a magnetic field of symmetrical distribution, themagnetic field being produced by a current carrying coil positionedaround the beam. The focusing of electron beams is required, forexample, in electron beam cutting or welding apparatus, in theilluminating system of electron microscopes of the transmission type, inscanning electron microscopes and in electron probe X-ray micro-analysisapparatus. In all these forms of apparatus, but particularly in the lastnamed form of micro-analysis apparatus an electron beam is required tobe focused on a small target area. It is desirable that the area inwhich the beam is concentrated should be very small and compact, and itis therefore'essential that the aberrations of the lens should be assmall as possible.

Magnetic lenses used hitherto have been positioned around the path ofthe beam and may occupy a considerable distance along the beam. Suchlenses so positioned can impose undesirable structural and designlimitations on the apparatus in which they are used. Thus, for example,in electron microscopes, magnetic condenser lens systems are employedwhich are closely positioned around the electron beam so that a verypowerful pumping system is required to maintain the desired vacuumconditions within the constricted passageways.

X-ray micro-analysis depends on the detection of X-rays emitted from theregion of a specimen bombarded by electrons, the X-rays beingcharacteristic of elements present at the region of bombardment. TheX-rays are emitted in all directions relative to the surface of thespecimen, from nonnal almost to a grazing angle. It has been acharacteristic of previously known electron probe X-ray micro-analysisapparatus that the X-ray emission has been intercepted over anappreciable solid angle by the lens structure through which the electronbeam has had to pass to be focused on the specimen. This has caused lowsensitivity of detection of the emitted X- rays, and mechanicaldifficulties in the placing of devices for viewing the specimen and ofX-ray spectrometers for analyzing the emitted X-rays into their variouscomponents.

\ The present invention provides a magnetic lens system for focusing abeam of charged particles which can have highly satisfactory opticalproperties, in particular low spherical aberration, and which can be soshaped and positioned with respect to the target position thatconsiderable improvements can be achieved in the overall design ofapparatus using existing magnetic lens systems.

The present invention is based on the discovery that a magnetic lenssystem having highly satisfactory optical properties can be provided bya conducting wire coil positioned close to the target position relativeto its diameter, even when the coil is positioned behind the targetposition with respect to the beam of particles.

In accordance with the invention, an apparatus which produces a beam ofcharged particles is provided with a magnetic lens system for focusingthe beam onto a target position which comprises an electricallyconducting wire coil which is positioned around the axis of the beameither in front of or behind the target position by a working distancewhich is less than the mean diameter of the coil. The mean diameter ofthe coil is the mean between the inner and outer diameters of the coilwinding.

When the wire coil is positioned in front of but close to the targetposition as above defined, a desirably low value of spherical aberrationmay be achieved by constructing the coil with the difference between itsouter and inner annular radii at least twice its axial thickness andwith the outer radius of its annulus at least twice the inner radius.The axial thickness of a coil is the average distance through the actualcoil winding in an axial direction. Either or both of these conditionsmay be advantageously applied to a wire coil placed in a negativeposition, that is to say, a position behind the target position.

Such coils may be said to be in the form of thick discs or plates andalthough the coils have preferably flat circular faces, it should beunderstood that the coils may be shallowly coned or dished so that inposition they present a concave face to the source of charged particles.The conical half-angle should not be less than 75', preferably not lessthan although when the coil is behind the target position the halfanglemay usefully be as low as 60.

A magnetic lens in accordance with the invention is preferably a coilhaving a disc or even pancake shape with its outer radius at least threetimes its inner annular radius and the outer radius at least five timesits axial thickness. The whole of such a coil can be positioned close tothe target position either behind or in front of the target position andexert a powerful focusing action on the beam. The disc shaped coil canbe conveniently fitted into apparatus leaving the whole space betweenthe source of charged particles and the target position around the beamof charged particles, available for siting a radiation detector or anyother device or structure. One important practical advantage of sitingthe coil behind the target position is that the coil may be convenientlyplaced in cryogenic apparatus so that it is maintained at a temperatureat which it is superconducting, thus permitting very high energizingcurrents and hence very strong magnetic focusing fields to be employed.

According to an optional feature of the invention, the magnetic lens isin the form of a coil which has a layer of material of high magneticpermeability applied to the side remote from the source of chargedparticles. The layer of magnetic material may be continuous within theperiphery of the coil and preferably is provided with a removablecentral portion concentric with the coil so that the coil may bepositioned around the beam of charged particles.

The layer of magnetic material may also extend round the periphery ofthe coil and cover at least a portion of the side facing the source ofcharged particles.

According to a further optional feature of the invention, the magneticbacking provides an electrical connection to the center of the coil.

Although one important advantage of the disc shaped magnetic lens coilis that relatively high current densities may be used to produce strongmagnetic focusing fields as it has relatively large surfaces which maybe used to cool the coil, the coil may be cooled or further cooled bycooling means located, for example, within the layer of magneticmaterial or within the structure of the coil.

The invention will be further described, by way of example only, inrelation to the accompanying drawings in which:

FIGS. 1 and 2 are diagrammatic sectional views of apparatus showing therelative position of a lens coil, a target and a beam of chargedparticles;

FIG. 3 illustrates in section along its axis a lens including a magneticbacking for the coil and means for cooling the coil;

FIG. 4 illustrates in section along its axis an alternative form of coilconstruction and cooling there for, and an alternative construction ofthe magnetic backing;

FIG. 5 illustrates in section a mode of making a connection to the innerend of the coil;

FIG. 6 illustrates in section an arrangement for maintaining the coil ata temperature at which it is superconducting; and

FIG. 7 illustrates in section an embodiment of the invention in whichthe coil is positioned in front of the target.

Referring to FIG. 1, 11 represents diagrammatically a source of chargedparticles, hereinafter referred to for convenience as electrons,collimated by means not illustrated into a beam 12. Concentric with theaxis 'of beam 12, produced, and in a plane normal thereto, is situated acoil 13, having a front 14 facing the source 11 and a back 15. The coilis of generally annular shape, having an inner radius R1 an outer radiusR2 and an axial thickness T. The mean diameter MD is (R2 R1) and, asshown, is many times greater than the working distance W, the distancebetween the front of the coil and the focus F. By preferably making thedifference (R2 R1) between the inner and outer annular radii of the wirecoil at least twice its axial thickness T, the outer annular radius R2of the wire coil at least twice and preferably three times its innerradius R1, and the ratio of the outer radius R1 of the annulus of thewire coil not less than five times its axial thickness T it has beenfound that the magnetic lens system has highly satisfactory opticalproperties and can produce strong magnetic focusing fields which canfocus high energy electrons without bringing about excessive difficultywith dissipation of heat from the wire coil. Suitable excitation of thecoil 13 will cause the beam 12 of electrons to be brought to focus at atarget position F, at a distance W, the working distance, in front ofthe inner boundary of the coil 13. A specimen 16; may be placed so thatF lies in its surface. The specimen may be microwelded or investigated,for example, as the object of electron microscopical examination or byelectron probe X- ray micro-analysis. It is for the latter purpose thatthe present invention is particularly advantageous, since there is verylittle restriction of the angle over which the emitted X-rays may bedetected, whereas, as explained above, there is considerable restrictionwith magnetic lenses of known form.

In FIG. 1 the coil has been shown substantially flat in shape, butsimilar properties are exhibited by coils of a shallow conical shape,e.g. as shown diagrammatically in FIG. 2. FIG. 2 also shows, in blockform, an X-ray detecting device, 38, receiving radiation along adirection indicated by the broken line 39, from the specimen 16. Thedetector 38 may be placed anywhere in the solid angle subtended at thefocus F by the periphery 40 of the coil 13, except, of course, for therelatively small solid angle occupied by the electron beam 12.

An improved performance of the lens may be obtained by providing thecoil, on the back face, with a plate of magnetic material, indicated by17 in FIG. 3. This magnetic material may, by way of example, be softiron, transformer steel or mumetal. By reducing the reluctance of themagnetic circuit through the coil it enables higher flux densities to beemployed and more energetic electron beams to be focused. It is possibleto reduce the reluctance further by extending the backing of magneticmaterial round the periphery of the coil through a ring 18 of magneticmaterial, and, if desired, over the front face of the coil in the formof an annular plate 19. In FIG. 3 the plate 19 has been shown with thesame inner radius as that of the coil, but the inner radius of theannular plate may be greater than this.

It may be convenient from other design consideration for the backing ofmagnetic material to be tapered in thickness towards the rim, as shownat 20 in FIG. 4. This presents no magnetic disadvantage since in a plateof uniform thickness the flux density will be considerably greater nearthe middle than at the rim. The plate may be radially tapered so as tomaintain the flux density substantially constant through the material ofthe plate.

The distribution of the magnetic field strength along the coil axis mayconveniently be described by reference to the halfwidth. This is thedistance, measured along the coil axis, between the points on thedistribution curve at which the field strength is half the maximum fieldstrength. For the coil configuration herein described, the half-width ofthe axial magnetic field distribution is typically in the range 2R1 to10R1, where R1 is the inner radius of the coil as indicated in FIG. 1.

It is desirable that the magnetic field produced by the coil 13 be assymmetrical as possible. Any disturbance of symmetry is most likely toarise with the connection to the inner radius of the coil, and FIG.illustrates one way in which this may be made. Between the coil 13 andthe magnetic backing 17 may be placed a thin conducting layer 22 whichmay consist, e.g. of copper foil. To this may be connected, bysoldering, for example, the lead 23 to the inside of coil 13. Connectionto the foil may be made at a number of equally spaced locations aroundthe periphery, which approximates to that of the magnetic backing, sothat the flow of current to the inner connection to the coil may berendered substantially symmetrical with respect to the coil axis. Theconnection to the outside of the coil has a negligible influence on thesymmetry of the magnetic field produced by the coil, at least in theregion near the axis, which is the most important.

FIG. 5 illustrates a coil wound with wire, but it is possible, as analternative, to construct a continuous spiral of insulated thin metaltape to form a coil of similar overall dimensions. If anodized aluminumtape is employed, there is no need for insulation over and above thatprovided by the oxide layer since the voltage drop from one turn to thenext is relatively small; The almost solid metallic coil winding soproduced promotes the removal of heat.

When the most intense magnetic fields are required, the necessarycurrent may produce considerable quantities of [heat within the coilstructure. In order to avoid damage, cooling is desirable. This may beeffected, by way of example, by means of passages in the magneticbacking through which cooling fluid may be made to flow. FIG. 3illustrates an example of such construction. The face 15 of the coil 13is bonded closely to the magnetic backing 17 so that heat flow betweenthem is promoted. This may be effected with, for example, an epoxy resinloaded with powdered metal or other substances of high thermalconductivity. In the magnetic backing is formed a series of passageways,24, having an inlet 25 and an outlet 26. The passages may be in the formof a spiral and are situated as near as practicable to the surface ofthe magnetic backing adjacent to the coil. The passages 24 may be in theform of tubes cast into the magnetic backing. As one alternative theycould be milled into the surface and covered with a thin sheet of bondedon material, which could either be magnetic, or if non-magnetic could beso thin as not appreciably to increase the reluctance of the magneticcircuit. It may be desirable to cool the coil from the front face aswell and this may be effected with a similar set of passages 27 havinginlet and outlet 28 and 29 respectively. The plate 19 in which thesepassages are formed may be wholly or partly of magnetic material, or maybe wholly of non-magnetic material. If desired, the plate 17 may also beof non-magnetic material if the design of the lens does not demand a lowreluctance path.

For lenses of the highest power it may be desirable to adopt a differentcoil construction in order to assist cooling to a greater degree, asillustrated in FIG. 4. The coil is wound in a number of concentricsections 30, offset from each other alternately in an axial direction soas to provide passages 31 between the coil sections. The coils aremounted alternately on annular plates 32, 33 and the whole is sealed byinner and outer tube members 34, 35 respectively. The passages 31 aresupplied with cooling liquid through inlet 36 and outlet 37. In order toavoid interference with the symmetry of the magnetic field produced bythe lens the inlet pipe 36 is made of nonmagnetic material. As in thisconstruction the cooling fluid is in contact with the coil sections itis desirable that it be an inert substance with good dielectricproperties. A suitable fluid is transformer oil.

Heating effects in the coil may be avoided altogether by working undersuperconducting conditions. In these conditions the mode of working inwhich an electron beam is brought to a focus between the electron sourceand the lens coil, as illustrated in FIG. 1, is particularlyadvantageous since the bulky cryogenic equipment does not obstruct theX-rays emitted from the specimen 16 as in the case when the lens coil issituated between the electron source and the electron beam focus as inmore conventional forms of lens.

Apparatus according to the invention, for working under super-conductingconditions may be arranged as illustrated diagrammatically and by way ofexample in FIG. 6. The source, 11, of charged particles, the specimen l6and the X- ray detecting device 38 are arranged within an evacuableenclosure 41, supported by plate 48, the electrical connections to thesource and detecting device not being shown. The coil 13, the electricalconnections to which are not shown, is sup ported from plate 49 a shortdistance below the base of enclosure 41 so that both faces may be keptin contact with liquid helium, 42, in a double-walled container 43, forwhich the plate 49 provides a cover. For working under super-conductingconditions the conducting wire of the coil is preferably made of analloy of niobium and zirconium or an alloy of niobium and titanium or ofniobium stannide (Nb Sn). The vessel 43 may be filled through pipe 46,and filling assisted by the provision of a vent pipe 47. Either or bothpipes may be connected to apparatus, (not shown) which functions tocollect and store for re-use, helium evaporating from the main body ofliquid 42. In order to reduce heat transfer to the vessel 43 and itscontents, that vessel may be immersed in liquid nitrogen 44 in an outerdouble walled container 45 for which the plate 48 forms a cover.

A specific example of the invention has the inner coil radius R1 3.3 cmand the outer radius R2 cm, so that R2/Rl 3. If the electron beam has anaccelerating voltage of KV, and the coil provides an mrnf of 2,700ampere turns, a working distance of minus 1 cm is obtained, in thelocation shown in FIG. 1; that is to say, the beam is brought to focusbetween the beam source and the focusing coil.

In a second specific example of the invention the inner coil radius R1 2cm and the outer radius R2 ll. 4 cm, so that R2/R1 5.7. if the electronbeam has an accelerating voltage of KV, and the coil provides an mmf of1,600 ampere turns, a working distance of minus 2 cm is obtained.

FIG. 7 illustrates an embodiment of the invention in which the wire coil13 is in front of the target 16. The coil is positioned around theelectron beam 12, which is brought to a focus at a target position F ata working distance W behind the inner boundary of the coil 13. it hasbeen found that when the wire coil is arranged so that the workingdistance W is less than the mean diameter MD of the coil, while thedifference (R2 R1) between its outer and inner annular radii is at leasttwice its axial thickness T and the outer radius R2 of its annulus is atleast twice the inner radius R1 this magnetic lens system with the wirecoil in front of the target also has highly satisfactory opticalproperties and can produce strong magnetic focusing fields which canfocus high energy electrons.

The construction illustrated in FIG. 4, in which the wire coil isprovided with a magnetic backing, may be adapted to work, when required,according to the embodiment illustrated in FIG. 7 by the provision of aremovable portion 21, at the center of the coil backing, through whichthe electron beam 12 may pass to reach a focus at the far side of thecoil under suitable conditions of coil excitation and electron beamener- It may be convenient, in particular to promote dissipation of heatgenerated within the wire coil of the magnetic lens system, to constructthe coil so that its axial thickness tapers from the inner radiusoutward. The axial thickness T of the coil would then be defined as theaverage axial thickness. Likewise the conical angle of the coil would bethe average conical angle between the angles of the inner and outerconical faces of the coil.

We claim:

1. An apparatus which comprises a source arranged to produce a beam ofcharged particles and a magnetic lens for point focusing the beam onto atarget position located between said source and lens, the magnetic lenscoilwise consisting of electrically conducting wire coil meanspositioned around the axis of the beam and disposed fully behind thetarget position by a working distance which is less than the meandiameter of the coil to effect said point focusing onto said targetposition.

2. An apparatus according to claim 1 in which the difference between theinner and outer annular radii of the wire coil is at least twice itsaxial thickness.

3. An apparatus according to claim 1 in which the outer radius of theannulus of the wire coil is at least twice its inner radius.

4. Apparatus according to claim 1 in which the outer radius of theannulus of the wire coil is at least three times its inner radius.

5. Apparatus according to claim 1 in which the outer radius of theannulus of the wire coil is not less than five times its axialthickness.

6. Apparatus according to claim 1 in which the coil has a layer ofmaterial of high magnetic permeability applied to the side remote fromthe source of charged particles.

7. Apparatus according to claim 6 in which the layer of magneticmaterial is provided with a removable central portion concentric withthe coil.

8. Apparatus according to claim 6 in which the coil has cooling meansfor cooling the coil located within the layer of magnetic material.

9. Apparatus according to claim 1 in which the coil has cooling meanslocated within the structure of the coil for cooling the coil.

10. Apparatus according to claim 1 which includes means for maintainingthe coil at a temperature at which it is superconducting.

11. Apparatus according to claim 10 in which the coil is positioned sothat it can be contacted by liquid helium over substantially the wholeof its surface.

1. An apparatus which comprises a source arranged to produce a beam ofcharged particles and a magnetic lens for point focusing the beam onto atarget position located between said source and lens, the magnetic lenscoilwise consisting of electrically conducting wire coil meanspositioned around the axis of the beam and disposed fully behind thetarget position by a working distance which is less than the meandiameter of the coil to effect said point focusing onto said targetposition.
 2. An apparatus according to claim 1 in which the differencebetween the inner and outer annular radii of the wire coil is at leasttwice its axial thickness.
 3. An apparatus according to claim 1 in whichthe outer radius of the annulus of the wire coil is at least twice itsinner radius.
 4. Apparatus according to claim 1 in which the outerradius of the annulus of the wire coil is at least three times its innerradius.
 5. Apparatus according to claim 1 in which the outer radius ofthe annulus of the wire coil is not less than five times its axialthickness.
 6. Apparatus according to claim 1 in which the coil has alayer of material of high magnetic permeability applied to the sideremote from the source of charged particles.
 7. Apparatus according toclaim 6 in which the layer of magnetic material is provided with aremovable central portion concentric with the coil.
 8. Apparatusaccording to claim 6 in which the coil has cooling means for cooling thecoil located within the layer of magnetic material.
 9. Apparatusaccording to claim 1 in which the coil has cooling means located withinthe structure of the coil for cooling the coil.
 10. Apparatus accordingto claim 1 which includes means for maintaining the coil at atemperature at which it is superconducting.
 11. Apparatus according toclaim 10 in which the coil is positioned so that it can be contacted byliquid helium over substantially the whole of its surface.